This invention relates to the upgrading of hydrocarbon residues to obtain valuable products therefrom. More specifically, the invention relates to a thermal process for upgrading petroleum residua.
It is important to find improved methods of processing petroleum residua because the quantity of high quality petroleum stock is dwindling. More of the higher value petroleum products must be obtained from the whole crude and in particular from the resid. It is essential to remove metal, nitrogen, sulfur and other contaminants from the petroleum residua since such contaminants greatly reduce the value of the coke produced or poison the catalysts used in various upgrading processes.
Various methods are used to upgrade heavy petroleum residua to products of higher value and saleability. These methods have included hydrogenative processing in which the residua is contacted with hydrogen in the presence of a hydrogenation catalyst to remove the metals, nitrogen, and sulfur compounds contained in the residua, and hydrotreating to saturate aromatic and asphaltene compounds contained therein. Another commonly used method of upgrading residua is solvent extraction such as deasphalting. In this process the residua is contacted with a solvent such as propane to precipitate the high molecular weight asphaltene fraction.
Another common technique for upgrading residua is thermal processing, for example, coking. In this thermal process the heavy hydrocarbon feed is heated rapidly to cracking temperatures so as to convert the feed into cracked vapors and solid coke which is produced at typical coking conditions in amounts of greater than about 20 wt. % based of feed. Another thermal process is thermal visbreaking, a relatively mild cracking process in which the higher molecular weight components in the resid are cracked to lighter products.
Visbreaking promotes condensation reactions which yield carbonaceous materials including toluene insolubles which are incompatible with the remaining products and are undesirable. Under visbreaking conditions, these toluene-insoluble compounds remain in a liquid phase. These toluene-insoluble compounds are physically indistinct at the macro level from the remainder of a visbreaker product stream. Thus, the visbreaker product forms a single stable liquid phase which cannot be separated by decantation. The insolubles are even less desirable than solid coke in that they have little or no commercial use. Visbreaking conditions, therefor, are generally controlled to limit the formation of toluene insolubles to less than 0.5 wt. % based on feed.
The term "toluene-insolubles" as used herein is not the equivalent of "solid coke". The toluene-insolubles fraction is a complex hydrocarbon mixture soluble in solvents such as pyridine and tetrahydrofuran, and nominally has a hydrogen content of about 6-7 weight percent. Pyrolysis of a toluene-insolubles fraction yields a solid coke residue which represents substantially less (up to 30 to 50 percent less) than the original toluene-insolubles weight.
Hydroprocessing techniques such as those discussed above employed to upgrade resid require high hydrogen pressures, low flow rates and high temperatures. High capital investment and operation costs are thus necessitated. Solvent extraction using light hydrocarbons as solvents generally lacks selectivity for the contaminants and thus high ratios of solvent to residua requirements are required. The requirement for large amounts of solvent increases the capital investment cost as well as operational costs and energy consumption. The poor selectivity of the solvent extraction process leads to the production of a large volume of low value asphaltene. Disposal of this asphaltene can itself become a problem.
Coking to upgrade the petroleum residua is becoming more and more popular in the petroleum industry due to the need for minimizing residual fuel production. However, coking is both capital and operation cost intensive. Handling of solid coke including drilling, blowing-down and transportation is expensive. A substantial amount of solid coke, 20 to 40% by weight of resid, depending on the nature of the resid, is produced in the process. Coke yields generally range between 100 and 200% of the resid's ASTM D-189 Conradson Carbon Residue.
Due to the high levels of metal and sulfur contamination, the solid coke from low quality resids is often sold as fuel coke at about $20 per ton. Furthermore, a low quality resid often yields shot coke. Shot coke is currently valued at less than $10 per ton. To put these prices in perspective, it is interesting to note that the prices of crude and resids are $200 per ton and $150 per ton, respectively.
There are numerous patents related to the coking of heavy petroleum feeds including petroleum residua. Typical patents include U.S. Pat. No. 3,247,095 and 4,394,250. The latter patent also discloses adding small amounts of cracking catalyst and hydrogen to the hydrocarbon feedstock before it is charged to the coking drum. In U.S. Pat. No. 4,522,703 solid carbonaceous fines are incorporated into the feedstock which is subsequently converted to gasoline, gas oil, and resid tar. U.S. Pat. No. 4,522,703 is incorporated herein by reference.
Accordingly, it is an objective of the present invention to provide a less expensive process as an alternative to the conventional methods for upgrading resids. A further objective is to maximize the yield of upgraded liquid product without the use of external hydrogen. Another objective is to minimize the yield of solid coke and gas in the coking process. Still another object is to provide a continuous process thereby eliminating the need for a batch or semi-batch process. Other objectives of the present invention will become apparent from the accompanying description and illustrated Example.